This invention relates generally to non-destructive examinations of gas turbine engine components, and more particularly to non-destructive evaluations of machining induced surface defects on gas turbine engine components.
A gas turbine engine includes a compressor for compressing air which channeled to a combustor and mixed with a fuel, wherein the mixture is ignited for generating hot combustion gases. The combustion gases are channeled to a turbine, which extracts energy from the combustion gases for powering the compressor, as well as producing useful work to propel an aircraft in flight or to power a load, such as an electrical generator. Increased efficiency in gas turbine engines is facilitated at least in part by an increase in the operating temperature of the combustor.
Machining process, for example, broaching on gas turbine engine components can produce a surface damaged metallurgical layer. This surface damaged layer can result in reduced low cycle fatigue (LCF) lives. Machining induced surface damaged layers are typically measured with optical metallography. These damaged layers are about 0.001 inch (25 micrometers (μ)) or less in thickness, which approaches the accuracy limit of optical metallography. This is especially true at free surfaces which suffer from rounding during metallurgical sample preparation. It has been shown that optical metallography measurements can vary from about 50 percent to 150 percent of the depth of the measurement.
Machining of turbine components causes a distortion of the microstructural features immediately adjacent to the machined surface. This distortion can vary in degree of damage from minor cold working, exhibited by curved crystallographic grain boundaries in the direction of cold work, to a refined crystallographic grain structure that appears as a distinguishable layer at the material surface. The thickness and severity associated with the cold worked and refined structures have been seen to change with machining process parameters. It is generally accepted that an increase in the severity and thickness of these layers will affected the low cycle fatigue life of the turbine component.